Further increase in the number of component parts per semiconductor device has been desired in recent years, and the formation of further minute patterns has been required accordingly. To meet such a requirement, a photolithographic process needs to form a thin resist film on a semiconductor wafer to form a minute pattern of a high resolution by a dry etching process.
When a thin resist film is used to form a minute pattern, a film to be etched cannot be etched in a satisfactory profile. Because an etching selectivity of the film to be etched relative to the resist film is not enough.
A conventional technique uses a multilayer resist film to solve such a problem. A multilayer resist film, for example, consists of a lower resist film of an organic material formed on a film to be etched, a SOG film (spin-on-glass film), namely, an inorganic film, formed on the lower resist film, and an upper resist film of a photoresist formed on the SOG film.
The upper resist film, namely, the photoresist film, is subjected to exposure and developing processes to form a patterned photoresist film, the SOG film underlying the patterned photoresist film is etched by using the patterned photoresist film as a mask, and then the organic lower resist film is etched by using the etched SOG film.
It has been usual to etch the organic lower resist film underlying the SOG film by using the SOG film as a mask in a N2/O2 atmosphere. However, such an etching method could not etch the organic lower resist film at a satisfactorily high etch rate.
A technique disclosed in JP 1-280316 A intended to solve such a problem uses an etching gas containing NH3 gas for etching such a multilayer resist film to etch the multilayer resist film at a high etch rate.
The technique disclosed in JP 1-280316 A, however, is incapable of achieving etching at a high etch rate for some combination of an etching gas and a pattern, the controllability of CD shift is unsatisfactory, the film is etched in a bowing shape, and in-plane etch uniformity is unsatisfactory. More over, the SOG film is liable to come off when the pattern has a small opening ratio, i.e., the ratio of the area of openings formed in a film to the total area of the film. These problems arise not only in etching a multilayer resist film, but also in etching an organic film by using an inorganic film as a mask.